Single atom catalyst and method of forming the same

ABSTRACT

A single atom catalyst and a method of forming the same are provided. The single atom catalyst comprises a support comprising a first metal oxide and a second metal atom located in the first metal oxide. The method of forming the single atom catalyst comprises forming a sacrificial nanoparticle, coating the sacrificial nanoparticle with a first metal oxide, adsorbing a second metal atom to the first metal oxide, forming a sacrificial layer on the support, and heating the first metal oxide.

TECHNICAL FIELD

The present invention relates to a single atom catalyst and a method of forming the same.

BACKGROUND ART

The reversible and cooperative activation processe, which includes electron transfer from surrounding redox mediators, reversible valence change of cofactors, and macroscopic functional/structural change, is one of the most important characteristics of biological enzymes, and has been frequently used in designing homogeneous catalysts. However, There is nearly no report on industrially important heterogeneous catalysts with these enzyme-like characteristics.

Heterogeneous photocatalysts have many potential applications such as hydrogen production, CO₂ conversion, water treatment and organic synthesis. In order to achieve high efficiency and selectivity in these applications, the electronic band structures of cocatalysts and their interactions with light absorbers should be investigated, along with the intrinsic light-absorbing properties of the photocatalysts. However, it is difficult to understand the mechanism during photocatalytic reaction atomically because the position and valence of the cocatalysts are difficult to control at the atomic level.

DISCLOSURE Technical Problem

In order to solve the above mentioned problems, the present invention provides a single atom catalyst having good performance.

The present invention provides method of forming the single atom catalyst.

The other objects of the present invention will be clearly understood by reference to the following detailed description and the accompanying drawings.

Technical Solution

A single atom catalyst according to the embodiments of the present invention comprises a support comprising a first metal oxide and a second metal atom located in the first metal oxide.

A method of forming a single atom catalyst according to the embodiments of the present invention comprises forming a sacrificial nanoparticle, coating the sacrificial nanoparticle with a first metal oxide, adsorbing a second metal atom to the first metal oxide, forming a sacrificial layer on the support, and heating the first metal oxide.

Advantageous Effects

A single atom catalyst according to embodiments of the present invention may have good performance. The single atom catalyst can achieve uniquely improved catalytic performance by adjusting local atomic composition for a single atom fixed to a support. In addition, the single atom catalyst may have good photocatalytic properties. The single atom catalyst can be easily formed in a simple way.

DESCRIPTION OF DRAWINGS

FIG. 1 shows images of Cu/TiO₂ at various states of the photocatalysis cycle.

FIG. 2 shows the photocatalysis cycle of Cu/TiO₂.

FIG. 3 shows candidate binding sites for single metal atoms on the surface of TiO₂ anatase (101).

FIG. 4 shows the Born-Haber thermodynamic cycle for calculating single atom catalyst formation energies.

FIG. 5 shows the Born-Haber energy components for possible single atom binding sites calculated using DFT (density functional theory).

FIG. 6 shows a formation process of a single atom Cu/TiO₂ photocatalyst.

FIG. 7 shows a TEM image of Cu/TiO₂.

FIG. 8 shows STEM-EDS elemental mapping of Cu/TiO₂.

FIG. 9 shows Cu K-edge XANES spectra of Cu/TiO₂.

FIG. 10 shows EXAFS spectra and analysis of Cu/TiO₂ at Ti and Cu K-edges.

FIG. 11 shows simulated HAADF-STEM image of anatase TiO₂.

FIGS. 12 and 13 are Cs-corrected HAADF-STEM image of anatase TiO₂.

FIG. 14 shows Fourier transform pattern of Rh/TiO₂.

FIG. 15 shows XY line scan profile of Rh/TiO₂.

FIG. 16 shows H₂ generation rate of Cut/TiO₂ depending on the loading amount of Cu.

FIG. 17 shows H₂ generation cycle of Cu/TiO₂.

FIG. 18 shows absorbance spectra change of Cu/TiO₂ before and after 10 min of light irradiation.

FIG. 19 shows Cu K-edge XANES spectra of Cu/TiO₂ before and after 10 min of light irradiation.

FIG. 20 shows H₂ evolution rate of Cu/TiO₂ depending time.

FIG. 21 shows photoluminescence spectra of Cu/TiO₂ before and after 10 min of light irradiation.

FIGS. 22 to 24 are views for explaining role of isolated Cu atoms in the cooperative interplay of Cu and TiO₂.

BEST MODE

Hereinafter, a detailed description will be given of the present invention with reference to the following embodiments. The purposes, features, and advantages of the present invention will be easily understood through the following embodiments. The present invention is not limited to such embodiments, but may be modified in other forms. The embodiments to be described below are nothing but the ones provided to bring the disclosure of the present invention to perfection and assist those skilled in the art to completely understand the present invention. Therefore, the following embodiments are not to be construed as limiting the present invention.

Terms like ‘first’, ‘second’, etc., may be used to indicate various components, but the components should not be restricted by the terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. A first element, component, region, layer or section could be termed a second element, component, region, layer or section without departing from the teaching of the embodiments of the present invention.

It will be understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements may be present therebetween. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. It is to be understood that the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. It will be further understood that the terms “comprises” or “has,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

A single atom catalyst according to the embodiments of the present invention comprises a support comprising a first metal oxide and a second metal atom located in the first metal oxide.

The second metal atom may be located in a first metal vacancy in the first metal oxide. The first metal oxide may comprise TiO₂. The second metal may comprise a transition metal. The second metal may comprise at least one of Cu, Fe, Co, Ni, and Rh.

The support may have a hollow spherical shape. The first metal oxide may have crystalline property.

The single atom catalyst may be activated by light irradiation and deactivated by exposure to oxygen.

A method of forming a single atom catalyst according to the embodiments of the present invention comprises forming a sacrificial nanoparticle, coating the sacrificial nanoparticle with a first metal oxide, adsorbing a second metal atom to the first metal oxide, forming a sacrificial layer on the support, and heating the first metal oxide.

The first metal oxide may comprise TiO₂. The second metal may comprise a transition metal. The second metal may comprise at least one of Cu, Fe, Co, Ni, and Rh. The first metal oxide may be changed from amorphous to crystalline by the heating. The second metal atom may be disposed in a first metal vacancy of the first metal oxide by the heating.

The sacrificial nanoparticle and the sacrificial layer may be formed with SiO₂.

The method may further comprise removing the sacrificial nanoparticle and the sacrificial layer.

[Formation Example of a Single Atom Catalyst]

A method of forming a single atom catalyst according to an embodiment of the present invention comprises forming a SiO₂ nanoparticle, coating the SiO₂ nanoparticle with TiO₂, adsorbing transition metal atoms to the TiO₂, and coating the TiO₂ with SiO₂, heating the TiO₂, and removing the SiO₂ nanoparticle and SiO₂ coating layer.

1) Forming a SiO₂ Nanoparticle

SiO₂ nanoparticles are formed. The SiO₂ nanoparticles can be formed by a sol-gel reaction. For example, the SiO₂ nanoparticles can be formed by adding TEOS (0.86 mL) to a solution containing ethyl alcohol (23 mL), H₂O (4.3 mL) and aqueous ammonia (0.6 mL) at room temperature, and vigorously stirring for about 6 hours. The SiO₂ nanoparticles are centrifuged, washed with water and ethyl alcohol, and then dispersed in ethyl alcohol. The SiO₂ nanoparticles may have a spherical shape.

2) Coating the SiO₂ Nanoparticle with TiO₂

The SiO₂ particles are coated with TiO₂. The nanoparticles are dispersed in 40 mL of anhydrous ethyl alcohol to form a SiO₂ nanoparticle solution. 14 mL of pure acetonitrile and 0.4 mL of aqueous ammonia (28-30 wt %) are added to the SiO₂ nanoparticle solution to form a first mixed solution. The amount of the aqueous ammonia in the first mixed solution affects the kinetics of the TiO₂ coating. The SiO₂ nanoparticles are well dispersed by sonicating the first mixed solution for 10 minutes. A second mixed solution is formed by dissolving 0.8 mL of TBOT (Titanium (IV) n-butoxide) in a mixed solution of 6 mL of anhydrous ethyl alcohol and 2 mL of acetonitrile. The first mixed solution and the second mixed solution are mixed and stirred for about 3 hours to coat the SiO₂ nanoparticles with TiO₂. The resulting white solution is centrifuged and washed with ethyl alcohol and water. The SiO₂ nanoparticles coated with TiO₂ (SiO₂@TiO₂ nanoparticles) are dispersed in 40 mL of H₂O.

3) Adsorbing Transition Metal Atoms to the TiO₂

The transition metal atoms are adsorb to the TiO₂. Metal chloride hydrates (FeCl₃·6H₂O, CoCl₂·3H₂O, NiCl₂·6H₂O, CuCl₂·2H₂O and RhCl₃·xH₂O) are used as metal precursors. 4.0 mg of metal chloride is added to 40 mL of a colloidal solution of the SiO₂@TiO₂ nanoparticles. The mixed colloidal solution is vigorously stirred at room temperature for 3 hours to adsorb the metal atoms to the TiO₂ of the SiO2@TiO2 nanoparticles. The SiO₂@TiO₂ nanoparticles to which the metal atoms are adsorbed (SiO₂@M/TiO₂ nanoparticles) are centrifuged and washed with water. Although the color of SiO₂@TiO₂ nanoparticles is white, the color of the nanoparticles changes due to the metal ion adsorption, and the color changes depending on the adsorbed metal atom (Fe: yellow, Co: blue, Ni: green, Cu: light blue, Rh: Orange).

4) Coating the TiO₂ with SiO₂

A SiO₂ coating layer is formed on the TiO₂ to which the transition metal atoms are adsorbed. The SiO₂@M/TiO₂ nanoparticles are dispersed in 40 mL of H₂O. PVP (0.4 g) is added and the solution is stirred overnight to adsorb PVP on the surface of SiO₂@M/TiO₂ nanoparticles. After PVP adsorption, the product is separated by centrifugation and redispersed in a solution of ethanol (46 mL) and H₂O (8.6 mL) by strong sonication for 10 min. Then, 1.2 mL of aqueous ammonia (28-30 wt %) and 1.6 mL of tetraethyl orthosilicate (TEOS) are added to the solution. Immediate stabilization of the adsorbed metal atoms on the surface causes a rapid color change within 10 seconds and forms a SiO₂ coating layer. After 4 hours of reaction, the SiO₂@M/TiO₂ nanoparticles having the SiO₂ coating layer (SiO₂@M/TiO₂@SiO₂ nanoparticles) are washed with ethanol and water. SiO₂@M/TiO₂@SiO₂ nanoparticles are centrifuged, dried in air at 80° C., and ground with a mortar to achieve uniformity.

5) Heating the TiO₂

The TiO₂ is heated. In order to spatially limit the redistribution of metal atoms, an annealing process in which dry SiO₂@M/TiO₂ nanoparticle powder is calcined at 900° C. for 2 hours is performed. It is preferable to supply sufficient oxygen in the annealing process. By the heating, the TiO₂ changes from amorphous to crystalline, and the transition metal atoms are disposed in the Ti vacancies.

6) Removing the SiO₂

The SiO₂ nanoparticle and the SiO₂ coating layer are removed. For SiO₂ etching, calcined SiO₂@M/TiO₂ nanoparticles are dispersed in 0.5 M NaOH solution. The solution is heated to 90° C. with continuous stirring. After 6 hours, the product is separated by centrifugation and washed with H₂O and ethyl alcohol to obtain M/TiO₂. The M/TiO₂ is dried in an electric oven at 80° C. Thereby, a single atom catalyst is formed. The TiO₂ functions as a support or co-catalyst of a single atom catalyst. In addition, the TiO₂ may have crystalline property and may have a hollow spherical shape.

FIG. 1 shows images of Cu/TiO at various states of the photocatalysis cycle, and FIG. shows the photocatalysis cycle of Cu/TiO₂.

Referring to FIGS. 1 and 2, the single atom Cu/TiO₂ catalyst according to an embodiment of the present invention is a site-specific single atom catalyst and undergoes a unique photoactivation process under photocatalytic H₂ generation reaction conditions. The color of Cu/TiO₂ changes from white to black after light irradiation in a 3:1 (v/v) water-methanol solution under Ar atmosphere. Black Cu/TiO₂ retains its color and efficiently generates H₂ even after the light is turned off under Ar atmosphere. When exposed to O₂ without light irradiation, the color returns to its original white color and the photoactivation cycle is completed.

The single atom Cu/TiO₂ catalyst initially has isolated copper atoms and TiO₂ in a dormant state and an inactive state (CTO state). By absorbing light generating electrons and holes, the CTO state is changed to a photo-excited state (CT1 state). The photo-generated electrons move from the conduction band of the TiO₂ to the d-orbital of the isolated copper atom. The extra charge is compensated by oxygen protonation, resulting in a valence change of the isolated copper atom of redox activity (CT2 state). The trapped electrons in the copper d-orbital induce a polarization field, resulting in localized TiO₂ lattice distortion around the isolated copper atom (CT3 state). The CT3 state exhibits completely different photoelectrochemical properties and greatly enhances the photocatalytic H₂ generating activity. The active CT3 state can easily revert to the original dormant CTO state upon brief exposure to O₂ for several minutes in the dark. This cooperative and reversible interaction between the isolated copper atoms and adjacent TiO₂ is fundamentally similar to enzymes and related biomimetic homogeneous catalysts, unlike traditional heterogeneous catalysts.

FIG. 3 shows candidate binding sites for single metal atoms on the surface of TiO₂ anatase (101) and FIG. 4 shows the Born-Haber thermodynamic cycle for calculating single atom catalyst formation energies. FIG. 5 shows the Born-Haber energy components for possible single atom binding sites calculated using DFT (density functional theory) and FIG. 6 shows a formation process of a single atom Cu/TiO₂ photocatalyst.

Referring to FIGS. 3 to 6, for the synthesis of site-specific single atom catalysts, candidate binding sites capable of stabilizing copper atoms on the TiO₂ anatase (101) surface are identified, the energetics involved in the binding process is determined, and density functional theory (DFT) can be used to design and synthesize exclusively single atom catalysts at vacancy-aided binding sites.

The formation energy (E_(F)) of the single atom catalysts can be simplified using the Born-Haber cycle which includes the binding site preparation energy (E_(p)) for a single atom and the single atom binding energy (E_(B)) of the prepared binding site (E_(F)=E_(p)+E_(B)). Based on the calculated energy component, candidate sites can be classified into sites requiring Ep and sites not requiring Ep. The three sites that do not require Ep (atop Ti site, hollow site and bridge site) are classified as surface binding sites, whereas the two binding sites that require Ep require vacancy-aided binding sites requiring high external energy for vacancy formation. The DFT results show that the synthesis must be controlled by thermodynamics to ensure binding at Ti vacancies rather than O vacancies. The surface is coated with a SiO₂ overlayer to prevent diffusion, and a high-temperature heat treatment is performed to incorporate metal atoms into only the most stable Ti vacancies on the TiO₂.

The single atom Cu/TiO₂ photocatalyst may be formed by 1) coating SiO₂ nanoparticles with TiO₂ and adsorbing metal atoms to the TiO₂ (Cu atom predistribution), 2) coating the TiO₂ with SiO₂ overlayer (Wrap process), 3) performing a heat treatment at 900° C. (Bake process), 4) etching and removing the SiO₂ (Peel process).

FIG. 7 shows a TEM image of Cu/TiO₂ and FIG. 8 shows STEM-EDS elemental mapping of Cu/TiO₂. FIG. 9 shows Cu Kedge XANES spectra of Cu/TiO₂ and FIG. 10 shows EXAFS spectra and analysis of Cu/TiO₂ at Ti and Cu K-edges. FIG. 11 shows simulated HAADF-STEM image of anatase TiO₂ and FIGS. 12 and 13 are Cs-corrected HAADF-STEM image of anatase TiO₂. FIG. 14 shows Fourier transform pattern of Rh/TiO₂ and FIG. 15 shows XY line scan profile of Rh/TiO₂.

Referring to FIG. 7, according to the TEM image, the well-dispersed Cu/TiO₂ nanoparticles comprise about 5 nm TiO₂ nanocrystal. Without protection by the SiO₂ coating layer, severe agglomeration may occur.

Referring to FIG. 8, energy dispersive X-ray spectroscopy (EDS) analysis in STEM mode shows a uniform dispersion of non-agglomerated copper species.

Referring to FIG. 9, X-ray absorption near edge structure (XANES) spectroscopy shows an absorption energy at 8996.3 eV and a small shoulder at 8989.3 eV in the initial white Cu/TiO₂ film, which are typical characteristics of Cu^(II).

Referring to FIG. 10, Ti K edge spectra from extended X-ray-absorption fine-structure (EXAFS) analysis show the existence of two characteristic distances, Ti-O and Ti-Ti, in the crystalline anatase phase. The EXAFS spectrum of the Cu K edge is very similar to the EXAFS spectrum of the Ti K edge, indicating that the metal atoms are located at the Ti site. The site-specific configuration results in not only a main peak corresponding to the direct binding of copper atoms and lattice oxygen (Cu-O), but also a minor peak indicating the local TiO₂ environment (Cu-Ti) around the isolated copper. The local coordination environment is investigated by EXAFS curve-fitting analysis. The best-fitting curve shows that the first peak originates from the first Cu-O shell coordination, whereas the minor second peak originates from the Cu-Ti contribution.

Referring to FIGS. 11 to 15, according to the method of forming a single atom catalyst of the present invention, it is possible to form single atom catalysts that are highly loaded with atomically dispersed metals including Cu, Fe, Co, Ni and Rh exclusively in the Ti vacancies of the hollow TiO₂ nanoparticles. The bright contrast spots are located only on the Ti atomic row, confirming that the Rh atoms are exclusively present in the Ti vacancies. In addition, the atomic resolution HAADF-STEM imaging and EXAFS analysis results are consistent and show the homogeneous incorporation of metal atoms in the site-specific Ti vacancies.

FIG. 16 shows H₂ generation rate of Cu/TiO₂ depending on the loading amount of Cu and FIG. 17 shows H₂ generation cycle of Cu/TiO₂. FIG. 18 shows absorbance spectra change of Cu/TiO₂ before and after 10 min of light irradiation and FIG. 19 shows Cu K-edge XANES spectra of Cu/TiO₂ before and after 10 min of light irradiation. FIG. 20 shows H₂ evolution rate of Cu/TiO₂ depending time and FIG. 21 shows photoluminescence spectra of Cu/TiO₂ before and after 10 min of light irradiation.

Referring to FIGS. 16 and 17, the H₂ production rate of the Cu/TiO₂ photocatalyst containing 0.75 wt % copper was the highest as 16.6 mmol/gh. The photocatalytic activity did not decrease significantly and remained stable while H₂ production was carried out for 4 consecutive cycles. The optimized 0.75 wt % Cu/TiO₂ photocatalyst shows an apparent quantum efficiency (AQE) of 45.5% at 340 nm, far exceeding the apparent quantum efficiency of conventional TiO₂-based photocatalysts.

Referring to FIG. 18, as revealed by the difference in color between the resting state sample and the active state sample, the UV-vis absorption spectra show totally different patterns. The resting state sample shows a sharp onset at about 390 nm and a broad band at 700 nm corresponding to the band-band transition of anatase TiO₂ and the d-d transition of Cu, respectively (black curve). The active-state sample shows very strong absorption in the whole range of measured wavelengths (300-800 nm) (red curve).

Referring to FIG. 19, the valence state change of isolated copper during the photoactivation process can be confirmed by XANES measurement. When the resting state sample is irradiated, a distinct shoulder at 8982.1 eV develops and the main peak at 8996.3 eV decreases, showing that the valence of the copper is reduced by photo-generated electrons. Considering the homogeneously distributed Cu atoms, the drastic change in absorption properties can be attributed to the cooperative photoactivation and subsedquential lattice distortion initiated by photo-generated electrons in between isolated copper atoms and local TiO₂ environment.

Referring to FIG. 20, the rate of H₂ evolution rapidly increased during the first 10 min of light irradiation and remained nearly constant afterwards.

Referring to FIG. 21, the photoluminescence spectrum of the initial resting state (CT0, black line) is almost identical to that of bare TiO₂ (blue line), showing that efficient separation of charge carriers does not occur before the light activation. The photoluminescence spectra of the active state (CT3, red line) significantly decreased, revealing that the initial white Cu/TiO₂ (CT0, resting state) is dormant and that the generated black Cu/TiO₂ (CT3, active state) has the exceptional photocatalytic activity.

FIGS. 22 to 24 are views for explaining role of isolated Cu atoms in the cooperative interplay of Cu and TiO₂.

Referring to FIGS. 22 and 23, DFT calculations elucidate that Cu atom in Ti vacancy provides mid-gap states with Cu dx²-y² and dz² characteristic in between the valence and conduction bands of TiO₂.

Referring to FIG. 24, after the photo-excitation (left of FIG. 24), the photo-generated electron is localized into the Cu dz² state and changes the valence state of Cu atom. An additional proton needs to be adsorbed on the surface to balance the surface charge (middle of FIG. 24). The photo-generated electron of the bare TiO₂ surface remains delocalized in the conduction band of TiO₂, rather than being localized at the metal center due to the absence of Cu redox center. Since the Cu dz² state has an axial anti-bonding characteristic, electron localization at this state leads to a lattice distortion by elongating of the backside oxygen coordination from 2.005 Å to 2.321 Å (right of FIG. 24), which not only stabilizes the localized electron at Cu, but also causes lattice distortion in nearby TiO₂. Consequently, isolated copper acts as a redox active metal cofactor that reversibly tune local TiO₂ lattice during dynamic photocatalysis. During the dynamic photocatalytic process, the valence state of the isolated Cu atoms is changed by the atomistic localization of photo-generated electrons. This valence change induces the activation of adjacent TiO₂, thereby tuning the initially dormant TiO₂ to active state that significantly enhance photocatalytic performance.

Although the embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that the present invention may be embodied in other specific ways without changing the technical spirit or essential features thereof. Therefore, the embodiments disclosed in the present invention are not restrictive but are illustrative. The scope of the present invention is given by the claims, rather than the specification, and also contains all modifications within the meaning and range equivalent to the claims.

Industrial Applicability

A single atom catalyst according to embodiments of the present invention may have good performance. The single atom catalyst can achieve uniquely improved catalytic performance by adjusting local atomic composition for a single atom fixed to a support. In addition, the single atom catalyst may have good photocatalytic properties. The single atom catalyst can be easily formed in a simple way. 

1. A single atom catalyst comprising: a support comprising a first metal oxide; and a second metal atom located in the first metal oxide.
 2. The single atom catalyst of claim 1, wherein the second metal atom is located in a first metal vacancy in the first metal oxide.
 3. The single atom catalyst of claim 1, wherein the first metal oxide comprises Tice .
 4. The single atom catalyst of claim 1, wherein the second metal comprises a transition metal.
 5. The single atom catalyst of claim 4, wherein the second. metal comprises at least one of Cu, Fe, Co, Ni, and Rh.
 6. The single atom catalyst of claim 1, wherein the support has a hollow spherical shape.
 7. The single atom catalyst of claim 1, wherein the first metal oxide has crystalline property.
 8. The single atom catalyst of claim, 1, wherein the single atom catalyst is activated by light irradiation and deactivated by exposure to oxygen.
 9. A method of forming a single atom catalyst comprising: forming a sacrificial nanoparticle; coating the sacrificial nanoparticle with a first metal oxide; adsorbing a second metal atom to the first metal oxide; forming a sacrificial layer on the support; and heating the first metal oxide.
 10. The method of claim 9, wherein the sacrificial nanoparticle and the sacrificial layer is formed with SiO₂.
 11. The method of claim 9, wherein the first metal oxide comprises TiO₂.
 12. The method of claim 9, wherein the second metal comprises a transition metal.
 13. The method of claim, 12, wherein the second metal comprises at least one of Cu, Fe, Co, Ni, and Rh.
 14. The method of claim 9, wherein the first metal oxide is changed from amorphous to crystalline by the heating.
 15. The method of claim 9, wherein the second metal atom is disposed in a first metal vacancy of the first metal oxide by the heating.
 16. The method of claim 9, further comprising removing the sacrificial nanoparticle and the sacrificial layer. 